As one of a vehicle steering control devices, a steering control device has been well known, which has an electric power steering unit which generates a steering assisting power for driving steerable wheels to steer and a steering ratio varying unit which drives an output member relative to an input member driven by steering operation of a driver to vary a steering ratio.
As is described in Japanese Patent Laid-Open Publication No. 2001-287657, it has already been known that in a vehicle having a common power source which supplies electric power to both an electric power steering unit and a steering ratio varying unit, a steering ratio is decreased and the steering ratio varying unit is stopped when the power source voltage falls below a predetermined value so as to prevent the electric power steering unit and the steering ratio varying unit from being unable to operate due to the drop of the power source voltage
In general, a steering assisting power (assist torque) which an electric power steering unit can generate is finite and decreases as the steering drive velocity of steerable wheels increases. In other words, the maximum value of the steering drive velocity range of steerable wheels under the situation where an electric power steering unit can generate a steering assisting power which it is demanded to generate decreases as a steering assisting power increases.
For that reason, even under the situation where the power source voltage is well above a predetermined value, if the output member is driven to greatly increase the rotation velocity relative to the input member or a rapid steering operation is conducted by a driver with the result that the steering drive velocity of steerable wheels becomes high, there arises a fear of a so-called power assistance failure in which a required steering assisting power can not be generated. That is, there is a fear that a steering assisting power which is required for the power steering unit becomes relatively larger than the steering assisting power which the power steering unit can generate for the reason that the steering drive velocity of steerable wheels increases.
It is to be noted that the problem of a power assistance failure not only arises in the case where the steering drive velocity of steerable wheels increases, but arises in the case where a steering assisting power which is required for the power steering unit increases. The problem of a power assistance failure arises even under the situation where the power source voltage is well above a predetermined value. Therefore, even if a predetermined value of the power source voltage is set high in the steering control device described in the above-mentioned Laid-Open Publication, the problem of a power assistance failure due to the increase of the steering drive velocity of steerable wheels can not be solved.
If a power steering unit which can generate a high steering assisting power is used, the possibility of a power assistance failure even under the situation where the steering drive velocity of steerable wheels or the required steering assisting power becomes high. In that case, however, a power steering unit of large and high power type is essential and a power source of high voltage and high capacity type is needed. Accordingly, the size and cost of the steering control device are inevitably increased and the embarkation capability of the steering control device is decreased.